For many years metal ingots, particularly aluminum or aluminum alloy ingots, have been produced by a semi-continuous casting process known as direct chill casting. In this procedure molten metal has been poured into the top of an open ended mould and a coolant, typically water, has been applied directly to the solidifying surface of the metal as it emerges from the mould.
Such a system is commonly used to produce large rectangular-section ingots for the production of rolled products, e.g. aluminum alloy sheet products. There is a large market for composite ingots consisting of two or more layers of different alloys. Such ingots are used to produce, after rolling, clad sheet for various applications such as brazing sheet, aircraft plate and other applications where it is desired that the properties of the surface be different from that of the core.
The conventional approach to such clad sheet has been to hot roll slabs of different alloys together to “pin” the two together, then to continue rolling to produce the finished product. This has a disadvantage in that the interface between the slabs is generally not metallurgically clean and bonding of the layers can be a problem.
There has also been an interest in casting layered ingots to produce a composite ingot ready for rolling. This has typically been carried out using direct chill (DC) casting, either by simultaneous solidification of two alloy streams or sequential solidification where one metal is solidified before being contacted by a second molten metal. A number of such methods are described in the literature that have met with varying degrees of success.
In Binczewski, U.S. Pat. No. 4,567,936, issued Feb. 4, 1986, a method is described for producing a composite ingot by DC casting where an outer layer of higher solidus temperature is cast about an inner layer with a lower solidus temperature. The disclosure states that the outer layer must be “fully solid and sound” by the time the lower solidus temperature alloy comes in contact with it.
Keller, German Patent 844 806, published Jul. 24, 1952 describes a single mould for casting a layered structure where an inner core is cast in advance of the outer layer. In this procedure, the outer layer is fully solidified before the inner alloy contacts it.
In Robinson, U.S. Pat. No. 3,353,934, issued Nov. 21, 1967 a casting system is described where an internal partition is placed within the mould cavity to substantially separate areas of different alloy compositions. The end of the baffle is designed so that it terminates in the “mushy zone” just above the solidified portion of the ingot. Within the “mushy zone” alloy is free to mix under the end of the baffle to form a bond between the layers. However, the method is not controllable in the sense that the baffle used is “passive” and the casting depends on control of the sump location—which is indirectly controlled by the cooling system.
In Matzner, German patent DE 44 20 697, published Dec. 21, 1995 a casting system is described using a similar internal partition to Robinson, in which the baffle sump position is controlled to allow for liquid phase mixing of the interface zone to create a continuous concentration gradient across the interface.
In Robertson et al, British patent GB 1,174,764, published 21 Dec. 1965, a movable baffle is provided to divide up a common casting sump and allow casting of two dissimilar metals. The baffle is movable to allow in one limit the metals to completely intermix and in the other limit to cast two separate strands.
In Kilmore et al., WO Publication 2003/035305, published May 1, 2003 a casting system is described using a barrier material in the form of a thin sheet between two different alloy layers. The thin sheet has a sufficiently high melting point that it remains intact during casting, and is incorporated into the final product.
Takeuchi et al., U.S. Pat. No. 4,828,015, issued May 9, 1989 describes a method of casting two liquid alloys in a single mould by creating a partition in the liquid zone by means of a magnetic field and feeding the two zones with separate alloys. The alloy that is fed to the upper part of the zone thereby forms a shell around the metal fed to the lower portion.
Veillette, U.S. Pat. No. 3,911,996, describes a mould having an outer flexible wall for adjusting the shape of the ingot during casting.
Steen et al., U.S. Pat. No. 5,947,184, describes a mould similar to Veillette but permitting more shape control.
Takeda et al., U.S. Pat. No. 4,498,521 describes a metal level control system using a float on the surface of the metal to measure metal level and feedback to the metal flow control.
Odegard et al., U.S. Pat. No. 5,526,870, describes a metal level control system using a remote sensing (radar) probe.
Wagstaff, U.S. Pat. No. 6,260,602, describes a mould having a variably tapered wall to control the external shape of an ingot.
It is an object of the present invention to produce a composite metal ingot consisting of two or more layers having an improved metallurgical bond between adjoining layers.
It is further object of the present invention to provide a means for controlling the interface temperature where two or more layers join in a composition ingot to improve the metallurgical bond between adjoining layers.
It is further object of the present invention to provide a means for controlling the interface shape where two or more alloys are combined in a composite metal ingot.
It is a further object of the present invention to provide a sensitive method for controlling the metal level in an ingot mould that is particularly useful in confined spaces.